You tell Audrey what's going down.

"Well, actually Audrey... It's Tim's birthday, so we thought we would surprise him with a special gift." Josh interrupts. "Well, I got him a gift right here!" She says as she displays the box. "Not that kind of gift Audrey, something more special." You say to her. "Well what then....?" She says. "Well... You two!" Josh says pointing at the both of you. Audrey looks confused and slightly shocked. "We are going to fuck him Audrey." You say so there will be no confusion. Audrey blushes a bright red. "You didn't tell me this was the plan!" She says, she sounds very upset. "Come on Audrey, it's the guys birthday and I will be right here with you... I know you like Tim too." You say trying desperately to convince her. "I do like Tim... I mean... if it's his birthday.... I guess I can do it." She says blushing even more. "Great! Now let's get ready" you say standing up.